This invention is directed to a charge accumulation and multiplication photodetector capable of sensing photons, accumulating charge in response to these photons, exhibiting internal gain and output of an enhanced displacement current signal for read out corresponding to the input photon flux.
A variety of photodetecting devices are known. These are utilized to produce electrical signals in response to optical signals. These range from single photodetectors utilized in optical communications as, for instance, in association with fiber optics to complex charge couple devices utilized as scene detectors for television and other video cameras, astronomical and military applications. For optical communication systems a variety of discrete avalanche photo diodes are known which utilize an avalanche multiplication technique to amplify light signals in the optical communication system.
With the exception of a recent thesis heretofore avalanching type devices useful for scene observation were not known. For charge couple devices and other array devices amplification of a photon flux has always been achieved external of the photodetector device and is accomplished by reading the charge output from the device with amplification of this charge then effected external of the device.
In a thesis entitled Avalanche Gain In Charge Coupled Devices, submitted to the Massachusetts Institute of Technology in August of 1986, Stephanie A. Gagar suggests the incorporation of an avalanche multiplication of charge on a charge coupled device. To accomplish this charge is collected and accumulated under a gate in a potential well. The accumulated charge is then transferred through an intermediate gate to a storage gate where it is temporarily stored. The original gate wherein the charge was first accumulated is then biased into avalanching. Charge is then transferred back from the temporary holding gate to the accumulating gate which is now biased as an avalanching gate. This is accomplished by pulsing the holding gate and spilling the charge from under the intermediate gate to the avalanching region. For further gain this procedure is repeated multiple times, i.e. 10 to 250 times, to build up charge. Once sufficient charge has been built up, the charge is moved off of the CCD gates to a charge sensitive amplifier for amplification and read out.
In this device, while gain was achieved it was also accompanied by increase in noise since during each avalanche "dark current" and other spurious charge is generated, added to signal charge and amplified. The author suggested but did not demonstrate that the dark currents and other spurious charges which led to amplified noise in the device might be eliminated by utilizing the device at a lower temperatures, i.e. below the minus 80.degree. C. temperature which the author utilized, or controlling spurious charge by changing the degree of ion implantation of channel stop regions or utilizing a fill plate for isolation.
Since all of the above described photodetection devices as well as other known photodetectors rely upon reading out charge, any device amplification as, for instance, avalanching also amplifies charges associated with noise and other spurious anomalies of the device which limits the resolution of the devices. Further, because off device amplifiers must be utilized to amplify the output charge of these devices operation of these devices is inherently more complex.
It is apparent that there is a need for new photon flux detecting devices which incorporate on device amplification and which do not rely upon read out of charge as the device output. Further, such devices should be capable of being constructed utilizing existing technology and design rules.